In conventional communication apparatuses, when wide band signals are transmitted and received in a mobile environment, frequency selective fading must be overcome. A multicarrier, in particular, orthogonal frequency division multiplexing (OFDM) is employed in various types of radio systems as a countermeasure technique for the frequency selective fading. On the other hand, attention is focused on multiple-input-multiple-output (MIMO) systems, which simultaneously transfer two or more signals by using multiple antennas, in order to further increase transmit capacity. MIMO systems are broadly classified into methods according to SDM and methods according to transmit diversity. A transmit diversity technique called space time coding (STC) belongs to the latter method.
An example of the SDM method is explained briefly (see “Proposal for SDM-COFDM for Wide Band Mobile Communication Realizing 100 Mbit/s by MIMO Channels”. Technical Report RCS2001-135 of The Institute of Electronics, Information and Communication Engineers). In a communication apparatus at a transmitter side, error correction coding is carried out individually on data of two channels that are transmitted simultaneously, and thereafter, a predetermined modulation processing is carried out on the respective data after coding, and the results thereof are placed at corresponding subcarriers. The signals on the respective subcarriers are then individually converted into time domain (OFDM signals) by inverse Fast Fourier Transform (IFFT) processing, guard intervals are added, the signals are upconverted to a high frequency band, and thereafter, are transmitted by corresponding transmit antennas.
In the communication apparatus at the receiver side, first, high frequency signals received at different reception antennas are individually converted into baseband signals. At this time, since multiple signals (the two channels) are mixed in each baseband signal, these signals must be separated. Each baseband signal is then converted into a frequency domain signal by Fast Fourier Transform (FFT) processing. Namely, the baseband signals become signals of subcarrier units (subcarrier signals). Since signals of plural channels are multiplexed, these subcarrier signals are extracted as reception signals of respective channels by weight control. “Proposal for SDM-COFDM for Wide Band Mobile Communication Realizing 100 Mbit/s by MIMO Channels”, Technical Report RCS2001-135 of The Institute of Electronics, Information and Communication Engineers, Zero-Forcing, which completely suppresses non-desired channels, is used in computing the weight. The reception signals, which are separated into channel units, are respectively subjected to metric computation by demodulation processing, and to error correction processing, and thereafter, are output as final reception signals of respective channels.
In this way, in a conventional communication apparatus that employs the SDM method, the number of transmission symbols per unit time can be increased by simultaneously transmitting different signal series by using plural channels. Namely, high-speed communication in a good channel condition can be realized.
On the other hand, in a communication apparatus that employs the STC method, generally, inverse matrix computation is not needed in channel separation at the reception side, and therefore, reception processing can be realized by a small amount of computation. Further, an apparatus configuration at the reception side can be realized by one antenna, and excellent communication quality can be ensured even in a low S/N environment. Theoretical signal processing of the STC method is disclosed in detail in S. M. Alamouti. “A Simple Transmit Diversity Technique for Wireless communications”, IEEE J. Selected Areas in Communications, vol. 16, pp. 1451-1458, Oct. 1998, and V. Tarokh, H. Jafarkhani, A. R. Calderbank, “Space-time Block Coding for Wireless Communications: Performance Results”, IEEE Journal On Selected Areas in Communications, Vol. 17, pp. 451-460, No. 3, March 1999.
However, in the conventional communication apparatus that employs the SDM method, since inverse matrix computation is needed for channel separation, the amount of computation increases. Further, for example, when an inverse matrix does not exist in the inverse matrix computation (or the matrix Eq. is near 0), a sudden deterioration in the signal-to-noise ratio (S/N) arises. Reception antennas in a number that is greater than or equal to the number of simultaneous transmission channels are needed.
In the conventional communication apparatus that employs the STC method, since the same signal is transmitted for plural times, it is difficult to increase the number of transmission symbols.
The communication apparatus that employs the STC method and the communication apparatus that employs the SDM method have mutually opposing features as described above. In other words, they have inherent problems. Therefore, there are more to be improved in structuring optimal MIMO channels.
The present invention has been achieved in order to solve the above problems, and it is an object of the present invention to provide a radio communication apparatus that can structure optimal MIMO channels by realizing the features of both methods and by realizing even higher speed.